Question

A beam of electrons is moving with constant velocity in a region having simultaneous perpendicular electric and magnetic fields of strength 20 vm^-1 and 0.5 T respectively at right angles to the direction of motion of the electrons. Then the velocity of electrons must be ______.

Options

• 8 m/s

• 20 m/s

• 40 m/s

• `1/40`m/s

Answer 1

A beam of electrons is moving with constant velocity in a region having simultaneous perpendicular electric and magnetic fields of strength 20 vm^-1 and 0.5 T respectively at right angles to the direction of motion of the electrons. Then the velocity of electrons must be 40 m/s.

Explanation:

The electron travels at a constant speed with no deflection. As a result, the force caused by an electric field and a magnetic field are equal and opposite.

qvB = qE

⇒ v = `"E"/"B"`

= `20/0.5`

= 40 m/s